Multi-phase signal generator and method

ABSTRACT

Multi-phase signal generators and methods for generating multi-phase signals are described. In one embodiment, the clock generator generates quadrature clock signals including those having 90, 180, 270 and 360 degrees phase difference with a first clock signal. One of the intermediate clock signals may be used as an enable signal to guide locking of all signals. For example, the 180 degree clock signal may be inverted and used as an enable signal to guide locking of the initial and 360 degree signals in a single phase adjustment procedure. The 0 and 360 degree signals may be delayed before their phase is compared to compensate for duty cycle error in the clock signals.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of pending U.S. patent application Ser. No. 12/245,444, filed Oct. 3, 2008, which application is incorporated herein by reference, in its entirety, for any purpose.

TECHNICAL FIELD

Embodiments of the invention relate generally to periodic signal generating systems, and more particularly, in one or more embodiments, to methods and systems for fast initializing a multi-phase clock signal generator.

BACKGROUND OF THE INVENTION

Periodic signals are used in a variety of electronic devices. One type of periodic signals are clock signals that can be used to establish the timing of a signal or the timing at which an operation is performed on a signal. For example, data signals are typically coupled to and from memory devices, such as synchronous dynamic random access memory (“SDRAM”) devices, in synchronism with a clock signal. More specifically, read data signals are typically coupled from a memory device in synchronism with a read data strobe signal. The read data strobe signal typically has the same phase as the read data signals, and it is normally generated by the same memory device that is outputting the read data signals. Write data signals are typically latched into a memory device in synchronism with a write data strobe signal. The write data strobe signal should have a phase that is the quadrature (having a phase 90-degrees relative to the phase) of the write data signals so that a transition of the write data strobe signal occurs during a “data eye” occurring at the center of the period in which the write data signals are valid.

Internal clock signals generated in electronic devices, for example, memory devices or memory controllers, are often synchronized or have some other controlled phase relationships relative to external or internal clock signals. For example, with reference to a memory device, a quadrature clock signal used for latching write data and outputting read data may be generated in the memory device to which the data are being written. The quadrature clock signal is typically generated in the memory device from an internal clock signal that is also derived from the system clock signal.

Internal clock signals having synchronized or some other controlled phase relationships with external and internal clock signals may also be used for applications other than for use as a write data and outputting read data strobe signal. For example, a “frequency doubler” circuit, which generates an output clock signal having twice the frequency of an input clock signal, can be implemented using an appropriate logic circuit that receives the input clock signal and quadrature versions of the input clock signal. Internal clock signals may also be generated having other than a quadrature phase relationships. Generally, any phase relationship between output clock signals can be used.

Various techniques can be used to generate a quadrature clock signal or read/write data strobe signal. If the frequency of the internal clock signal is fixed, quadrature clock signals can be generated by a timing circuit that simply generates a transition of the quadrature clock signals a fixed time after a corresponding transition of the internal clock signal. However, synchronous memory devices are typically designed and sold to be operated over a wide range of clock frequencies. Therefore, it is generally not practical to use a fixed timing circuit to generate quadrature signals from the internal clock signal. Instead, a circuit that can adapt itself to an internal clock signal having a range of frequencies must be used.

An example of such a circuit is a multi-phase clock signal generator. A multi-phase clock signal generator, as known, generates multi-phase clock signals to provide several clock signals having fixed phase relationships to a reference clock signal, such as an external or internal clock signal. In operation, a multi-phase clock signal generator should be initialized to ensure the generated clock signals have the correct phase relationship. A conventional method of initializing a multi-phase clock signal generator will now be described with reference to FIG. 1. The conventional multi-phase clock signal generator 100 includes a delay line 105 having a plurality of delay elements 110 a-d coupled in series with each other. Each of the delay elements 110 a-d has two inputs, two outputs, and a control input (not shown). Each delay element 110 a-d has two inputs and two outputs to provide for a double-ended configuration where both a clock signal 120 and its complement 121 are received and processed. A single-ended configuration may also be used.

Each of the delay elements 110 a-d couples a signal from its input to its output with a delay corresponding to a delay control signal applied to its control input. The input of the initial delay element 110 a receives a clock signal 120 and its complement 121. The outputs of all but the last delay element 110 d is coupled to the input of the subsequent delay element. The output of each delay element 110 a-d forms a respective tap of the delay line 105 to provide four clock signals C90, C180, C270, and C360, respectively, C360 is a one clock delayed version of C0 at lock status. As indicated by their names, the C90 signal has a 90 degree phase difference with the input clock signal 120. The C180 signal has a 180 degree phase difference with the input clock signal 120, the C270 signal a 270 degree phase difference, and the C360 signal a 360 degree phase difference. As explained in greater detail below, the amount of voltage-controlled delay provided by each of the delay elements 110 a-d sets a minimum and maximum amount of delay that can be achieved by the delay line 105.

To ensure the proper phase relationships are maintained correctly during operation between the four provided clock signals, a two-step locking phase detector 130 receives the input clock signal 120, the C180 signal and the C360 signal. The phase detector 130 will first lock the inversion of C180 signal to the C0 signal, and then in the second step, lock the C360 signal with the C0 signal. To lock the C0 and C180 signal, the phase detector 130 produces an error signal corresponding to a mismatch between the falling edge of the C180 signal and the rising edge of the C0 signal. The error signal is used to adjust the delay of the delay elements 110 a-d such that the C0 and C180 signals are 180 degrees apart. As shown in FIG. 1, the error signal is converted to a control signal by a control signal generator, such as charge-pump and loop filter 140. The control signal is used by a bias voltage generator 150 to couple a V_(BIAS) signal to the control inputs of the delay elements 110 a-d. In the second step of operation of the phase detector 130, an error signal is generated corresponding to a mismatch between a rising edge of the C0 signal and a rising edge of the C360 signal. In a similar manner, the error signal is used to adjust the delay of the delay elements 110 a-d. This two-step locking process may be sufficient in some cases where the duty cycle or slow locking time is not an issue. However, difficulties occur when the incoming clock signal contains some duty cycle distortion, as will now be explained with reference to FIG. 2.

FIG. 2 is a timing diagram illustrating signals from FIG. 1. A clock period is shown in FIG. 2 as t_(CK), between t₀ and t₂. The incoming clock signal, C0 has an amount of duty cycle distortion shown by t_(DCD). That is, in the case where the C0 signal had an ideal, 50 percent duty cycle, the high pulse would extend from time t₀ to time t₁ in FIG. 2. However, as shown, the C0 high pulse is significantly shorter. The phase detector 130 then locks the falling edge of the C180 signal with the rising edge of the C0 signal at time t₂, as shown by arrow 210. The phase detector 130 will lock the signals within a tolerance, shown by ±t_(PDmin) in FIG. 2. Due to the duty cycle distortion, the rising edge of the C180 signal is t_(x1) off from time t₁, where the signal should be for a 180 degree phase difference. Accordingly, the C180 signal has been delayed t_(x1) too much. Recall that adjusting the control voltage applied to the delay elements 105 of FIG. 1 adjusts the delay of all the delay elements 110 a-d. The C360 signal will now be 2*t_(x1) off from locked with C0, as shown in FIG. 2. The second step of operation of the phase detector 130 will be to adjust the delay of the delay elements 110 a-d such that the C360 signal is synchronized with the C0 signal, by matching the rising edge of the C0 signal with the rising edge of the C360 signal, as shown in the second timing diagram of FIG. 2 by the arrow 220.

Duty cycle distortion in incoming clock signals is not uncommon, and, taking signal jitter into consideration, could be a significant portion of reference clock period. With duty cycle distortion, the two-step locking phase detector 130 may cease to function properly. The delay line 105 may have insufficient range to accommodate the lengthy t_(x1) and 2*t_(x1) delay times that should be compensated for according to FIG. 2. One solution to this problem is to place a duty-cycle control element prior to and in series with the multi-phase clock signal generator 100. This may ensure the multi-phase clock signal generator receives a clock signal with a correct duty cycle. However, a duty cycle control element also has a limited working range and takes much longer time to achieve corrected output signals. Accordingly, this solution may also become impractical as speeds increase and timing requirements tighten.

There is accordingly a need for an improved system and method for providing multi-phase clock signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a multi-phase clock signal generator according to the prior art.

FIG. 2 is a timing diagram illustrating the operation of the clock signal generator of FIG. 1.

FIG. 3 is a schematic diagram of a multi-phase clock signal generator according to an embodiment of the present invention.

FIG. 4 is a timing diagram illustrating the operation of the signal generator of FIG. 3.

FIG. 5 is a schematic diagram of a multi-phase clock signal generator according to an embodiment of the present invention.

FIG. 6 is a timing diagram illustrating the operation of the signal generator of FIG. 5.

FIG. 7 is a schematic diagram of a memory device according to an embodiment of the present invention.

FIG. 8 is a schematic diagram of a processor-based system according to an embodiment of the present invention.

DETAILED DESCRIPTION

Certain details are set forth below to provide a sufficient understanding of embodiments of the invention. However, it will be clear to one skilled in the art that embodiments of the invention may be practiced without various of these particular details. In some instances, well-known circuits, control signals, timing protocols, and software operations have not been shown in detail in order to avoid unnecessarily obscuring the described embodiments of the invention.

An embodiment of a multi-phase clock signal generator 300 according to an embodiment of the present invention is shown schematically in FIG. 3. While one delay element per tap may be used in some embodiments of the present invention, as was described above with reference to FIG. 1, the delay line 305 of FIG. 3 includes twelve delay elements 310 a-1. Three delay elements are provided for each tap 320 a-d. Using multiple delay elements per tap may allow greater flexibility in the range of delay that can be provided by the delay line 305. For example, the bias generator 150 may produce one bias voltage, V_(BIAS), that adjusts each of the delay elements 310 a-1 the same amount. However, in some embodiments, the bias generator 150 may produce several bias voltages, shown as V_(BIAS1), V_(BIAS2), and V_(BIAS3) in FIG. 3. Each bias voltage may adjust a different set of delay elements 310 a-1. For example, delay elements 310 a, d, g, and j may be controlled by V_(BIAS1) and have a large range to provide course control of the delay of the delay line 305. Delay elements 310 b, e, h, and k may be controlled by V_(BIAS2) and have a medium range to provide medium range control of the delay of the delay line 305. Delay elements 310 c, f, i, and l may be controlled by V_(BIAS3) and may have a small range to provide fine control of the delay of the delay line 305. In this manner, a delay with adaptive wide range and timing delay resolution may be achieved.

Although four taps 320 a-d are shown in FIG. 3 to provide quadrature clock signals, any number of signals may be generated according to embodiments of the present invention having any phase relationship with the incoming signal, C0. Similarly, although three delay elements are shown in the delay line 305 for each tap 320 a-d, any number of delay elements may be used for each tap, including more than three delay elements and less than three delay elements.

The phase detector 330 of FIG. 3 is configured to provide an adjustment signal to lock the C0, C180 and C360 signals in a single adjustment, in contrast to the two-step process performed by the phase detector 130 of FIG. 1. The operation of the phase detector 330 will now be discussed with reference to the timing diagram of FIG. 4.

The phase detector 330 utilizes the C180 signal to guide the locking of the C0 and C360 signals. Accordingly, the C180 signal serves as an enable signal in that a phase difference between the C0 and C360 signals may be determined based on information gathered during a time period that the C180 signal is at a certain logic level. When the C180 signal is at a different logic level, the information is not gathered. On receipt of a transition of the C180 signal, the phase detector 330 initiates a lock of the C0 and C360 signals. By measuring the phase difference between the C0 and C360 signals and coupling an adjustment signal indicative of the phase difference to the control signal generator, the charge-pump and loop filter 140 as shown in FIG. 3. As described above, the charge-pump and loop filter 140 couples a V_(CONTROL) signal to the bias generator 150 to generate the V_(BIAS) signal applied to one or more of the delay elements 310 a-1 to adjust the delay of the delay line 305 to minimize the phase difference between the C0 and C360 signals. Accordingly, the solid lines in FIG. 4 indicate timing signals for one example of C0, C180, and C360 signals. As the solid line example shows, the three signals having a duty cycle distortion in that the high portion of each of the signals is less than 50 percent of the clock cycle t_(CK). Accordingly, C180 transitions low at time 405. This low transition can be used by the phase detector 330 to initiate the lock of C0 and C360. Accordingly, the phase detector 330 shortly detects the rising edge of C0 at time 410 followed by the rising edge of C360 at time 415. The phase detector 330 couples the adjustment signal indicative of the phase difference between the two signals to the charge pump and loop filter 140.

The dashed lines in FIG. 4 illustrate a case where the three signals have a duty cycle distortion causing the signals to be high for greater than 50 percent of the clock period. In this case, the falling edge of C180 that will enable the phase detector 330 occurs at time 420, after the rising edges of C0 and C360 at times 410 and 415, respectively. Accordingly the phase detector 330 does not obtain phase information regarding the rising edges of C0 and C360. When the C180 signal transitions high, the phase detector 330 is disabled, and the phase detector 330 accordingly does not obtain phase information regarding the rising edges of C0 and C360 at times 425 and 430, and will not be able to accurately lock the C0 and C360 signals. Accordingly, to accurately lock the C0 and C360 signals when the signals have a duty cycle error resulting in a high signal over 50 percent of the clock cycle, a duty cycle distortion tolerance delay element 340 is used, as shown in FIG. 3.

The duty cycle distortion tolerance delay element 340 delays C0 and C360 such that the rising edges of the C0 and C360 signals occur while the phase detector 330 is enabled. Alternatively, in some embodiments, the duty cycle distortion tolerance delay element 340 may delay the C180 signal such that the rising edges of the C0 and C360 signals occur while the phase detector 330 is enabled. The amount of delay provided by the duty cycle distortion tolerance delay element may be changed based on a frequency of the reference clock signal. In other embodiments, the amount of delay may be fixed. In FIG. 4, the amount of delay provided by the duty cycle distortion tolerance delay element 330 is shown as t_(DCD) _(—) _(Adjust). In the example of FIG. 4 the signals are shown containing duty cycle distortion, the C0 and C360 signals are delayed by the duty cycle distortion tolerance delay element 340. Accordingly, the rising edge of the delayed C0 signal occurs at time 450, after the falling edge of C180 at time 420 and the rising edge of the delayed C360 signal occurs at time 455, again after the falling edge of the C180 signal at time 420. In this manner, the phase detector 330 can accurately capture the phase difference between the C0 and C360 signals after being enabled by the falling edge of the C180 signal. Accordingly, the amount of delay provided by the duty cycle distortion tolerance delay element 340 may be selected based on the maximum duty cycle distortion anticipated in the C0 and C360 signals. In some embodiments, the duty cycle distortion tolerance delay element 340 provides a fixed amount of delay. In other embodiments, the delay provided by the duty cycle distortion tolerance delay element 340 is variable.

The above discussion describes a multi-phase clock signal generator using an intermediate signal as an enable signal to guide the locking of two of the generated signals. In the example described above, the 180 degree signal is used as an enable signal to facilitate the locking of the C0 and C360 signals. By locking the C0 and C360 signals, and providing delay elements 310 a-1 having equal amounts of delay between each of the generated signals, the C0, C90, C180, C270 and C360 signals will all accordingly be locked in a fast and accurate fashion. A specific example of circuitry used to accomplish the duty cycle distortion tolerance delay and phase locking will now be described with reference to the block diagram of FIG. 5 and the timing diagram of FIG. 6.

As shown in FIG. 5, the duty cycle distortion tolerance delay element 340 may be implemented by two delay elements receiving the C0 and C360 signals, respectively. The C180 signal is coupled to an inverter 505. The C0, C180, and C360 signals are shown in the timing diagram of FIG. 6. The inverted C180 signal is also shown with a rising edge at time 605. By inverting the C180 signal, the rising edge, at time 605, will serve as the enable signal for the phase detector 330 of FIG. 5. To represent the delay of signals C0 and C360 by the duty cycle distortion delay elements 340, the C180 signal is shown advanced an amount t_(DCD) _(—) _(Adjust). For purposes of illustration, advancing the C180 signal is equivalent to delaying the C0 and C360 signals. The advanced and inverted C180 signal accordingly has a rising edge at time 610, prior to the falling edges of C0 and C360 at times 620 and 615, respectively.

Two AND gates 510, 515 in FIG. 5 provide functionality for the phase detector 330. The first AND gate 510 provides a signal indicative of a phase difference between the C180 signal and the C0 signal by coupling a DN signal to the charge-pump and loop filter 140. The DN signal, as shown in FIG. 6, is high when both the inverted C180 signal is high and the delayed C0 signal is high. Accordingly, the DN signal is high from time 610 to time 620. The second AND gate 515 provides a signal indicative of a phase difference between the C180 signal and the C360 signal by coupling an UP signal to the charge-pump and loop filter 140. The UP signal, as shown in FIG. 6 is high when both the inverted C180 signal is high and the delayed C360 signal is high. Accordingly, the UP signal is high from time 610 to 615. The difference in the pulse widths of the DN and UP signals is indicative of the phase difference between the C0 and C360 signals. The DN and UP signals are coupled to the charge-pump and loop filter 140 as adjustment signals.

Note that, when the pulse widths of the UP and DN signals are equivalent, the C0 and C360 signals are locked, that is, the phase difference between the C0 and C360 signals is zero. Accordingly, the charge-pump and loop filter 140 are configured to adjust the delay based on a difference of pulse widths between the UP and DN signals to minimize the phase difference between the C0 and C360 signals. For example, the V_(CONTROL) signal may indicate to the bias generator 150 to increase the delay of the delay line 305 during a period when the UP signal is high and the DN signal low. The V_(CONTROL) signal may indicate to the bias generator 150 to decrease the delay of the delay line 305 during a period when the UP signal is low and the DN signal high. The V_(CONTROL) signal may indicate to the bias generator 150 to maintain the delay of the delay line 305 when the UP and DN signals have the same level. The charge-pump and loop filter 140 may be implemented in any of a variety of ways. For example, in one embodiment the loop filter may be implemented as a capacitance. The charge pump may include a current source which charges the capacitor responsive to the UP signal being high while the DN signal is low. The charge pump may further include a current sink which discharges the capacitor responsive to the UP signal being low while the DN signal is high. The charge pump would not effect the capacitor when the UP and DN signals had the same state. In this manner, the capacitor builds a voltage which may be, or be used to generate, the V_(CONTROL) signal.

FIG. 7 depicts a portion of a memory device 700. The memory device receives a clock signal CK 710 and complementary clock signal CKF 730. The CK and CKF signals may be coupled to the memory device 700 by a memory controller, processor, or other electronic element. The multi-phase clock signal generator 300 of FIG. 3 is coupled to a delay locked loop 705 for use in locking the output signals of the multi-phase clock signal generator to the received clock signal CK 710. The received clock signal 710 and optional complementary signal 730 are coupled to an input buffer 735. For example, the input buffer 735 may receive the clock signals 710 and 730 from off-chip, or from another portion of a chip than the input buffer 735. The input buffer 735 couples the buffered ClkRef signal to the delay lock loop 705. The delay lock loop 705 includes a delay line 740 and a phase detection and shift control element 745. The delay line is configured and controlled by the phase detection and shift control element 745 to output a clock signal CKi, and optionally a complementary clock signal CKiF that are in phase with the ClkRef signal. The phase detection and shift control element 745 couples a control signal 750 to the delay line 740 to adjust the delay of the delay line 740 to minimize a phase difference between the ClkRef signal and a feedback signal 755. The feedback signal 755 may be based on either one of the signals generated by the multi-phase clock signal generator 300 (C0 as shown in FIG. 7), or the input signal CKi to the multi-phase clock signal generator 300, as indicated by the dashed lines in FIG. 7. In this manner, the multi-phase clock signal generator 300 may be either inside of the delay-locked loop 705 (when the signal C0 is used as the feedback signal) or outside of the delay locked loop 705 (when the CKi signal is used).

FIG. 7 also illustrates the output signals of the multi-phase clock signal generator 300 (the signals C0, C90, C180, C270, and C360) coupled to a clock tree 715 for distribution to the DQ(s) 720 of the memory device 700. In this manner, the clock signals generated by the multi-phase clock signal generator may be used to clock operation of the DQ(s) 720. Although distribution to a DQ 720 is shown in FIG. 7, the clock signals from the multi-phase clock signal generator 300 may generally be coupled to any number of DQs. Further, output signals of the multi-phase clock signal generator 300 may additionally or instead be coupled to other elements of the memory device 700 or other electronic system employing the multi-phase clock signal generator 300. An output buffer 725 may be provided at each destination to couple one or more of the clock signals to the destination, such as DQ 720.

The feedback signal used by the phase detection and shift control element 745 may be coupled to one or more model delay elements, including the output model element 760 and the buffer delay element 765 shown in FIG. 7. The buffer delay element 765 models the delay of the input buffer 735. The output model delay element 760 models the delay of an output path between the point the feedback signal was generated, and the destination of the signal generated by the multi-phase clock signal generator 300 (the clock tree 715 and the output buffer 725 in the example of FIG. 7). By delaying the feedback signal by an amount equal to the delay of the output path, the delay locked-loop minimizes the phase difference between the output signal arriving at the DQ 720 and the input clock signal 710.

FIG. 8 is a block diagram of a processor-based system 1000 including processor 1002 that communicates with a memory device 700. The memory device 700 may be integral with or physically separate from the processor 1002 and communication between the two may take place in any manner. The memory device 700 may contain one or more multi-phase clock signal generators 300 to generate clock signals having various phases in accordance with embodiments of the invention described above. Typically, the processor 1002 is coupled through address, data, and control buses to the memory device 700 to provide for writing data to and reading data from one or more memory arrays in the memory device 700. The processor 1002 may include circuitry for performing various processing functions, such as executing specific software to perform specific calculations or tasks. In addition, the processor-based system 1000 includes one or more input devices 1004, such as a keyboard or a mouse, coupled to the processor 1002 to allow a user to interface with the processor-based system 1000. Typically, the processor-based system 1000 also includes one or more output devices 1006 coupled to the processor 1002, such as a printer or display. One or more data storage devices may also be coupled to the processor 1002 to store data or retrieve data from external storage media (not shown). Examples of such storage devices include hard and floppy disks, tape cassettes, compact disk read-only (“CD-ROMs”) and compact disk read-write (“CD-RW”) memories, and digital video disks (“DVDs”).

The processor-based system 1000 shown in FIG. 8 may be implemented in any of a variety of products employing processors and memory including for example cameras, phones, wireless devices, displays, chip sets, set top boxes, gaming systems, vehicles, and appliances. Resulting devices employing the processor-based system 1000 may benefit from the embodiments of a multi-phase clock signal generator described above to perform their ultimate user function.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, although the generation of quadrature clock signals has been discussed and described, embodiments of the invention may generate substantially any number of clock signals having any phase relationship therebetween by, for example, altering a number of identical delay elements per tap in the delay line generating the clock signals. Further, although analog embodiments are shown and described above, other embodiments of the present invention may be implemented using one or more digital components. 

What is claimed is:
 1. A signal generator comprising: a delay line configured to delay a first signal applied to an input of the delay line to generate a second signal, and further configured to delay the second signal to generate a third signal; a first tap coupled to the delay line and configured to receive the first signal, the first tap further configured to generate a first to signal based, at least in part, on the first signal; a second tap coupled to the delay line and configured to receive the third signal, the second tap further configured to generate a second tapped signal based, at least in part, on the third signal; a duty cycle distortion tolerance delay element configured to receive the first tapped signal and the second tapped signal, the duty cycle distortion tolerance delay element configured to delay the first and second tapped signals and output delayed first and second tapped signals; and a phase detector configured to receive the delayed first tapped signal, the second signal, and the delayed second tapped signal, the phase detector configured to be enabled responsive to the second signal and when enabled to provide a signal indicative of a phase difference between the delayed first tapped and delayed second tapped signals.
 2. The signal generator of claim 1, wherein the duty cycle distortion tolerance delay element is configured to delay the first and second tapped signals such that rising edges of the delayed first and delayed third signals occur while the phase detector is enabled.
 3. The signal generator of claim 1, wherein the duty cycle distortion tolerance delay element comprises a first delay element coupled to an output of the delay line and a second delay element coupled to an input of the delay line.
 4. The signal generator of claim 3, wherein the phase detector further comprises: an inverter coupled to receive the second signal and output an inverted second signal; a first AND gate configured to receive the inverted second signal and the delayed first tapped signal, the first AND gate configured to generate a first output signal when the inverted second signal and the delayed first tapped signal have a same logic level; and a second AND gate configured to receive the inverted second signal and the delayed tapped second signal, the second AND gate configured to generate a second output signal when the inverted second signal and the delayed tapped second signal have the same logic level.
 5. The signal generator of claim 4, further comprising a control signal generator configured to receive the signal indicative of the phase difference between the delayed first tapped and delayed second tapped signals and generate a control signal to adjust a delay of the delay line such that the first, second, and third signals have a predetermined phase relationship.
 6. The signal generator of claim 5, wherein the control signal generator comprises a charge pump and loop filter configured to receive the first and second output signals and generate the control signal to adjust a delay of the delay line to minimize a difference between the first and second output signals.
 7. The signal generator of claim 5, wherein the control signal generator is configured to adjust the phase relationship between the first and third signals and the phase relationship between the second and third signals in a single adjustment.
 8. The signal generator of claim 5, wherein the delay line includes a plurality of delay elements, individual ones of the plurality of delay elements configured to delay a signal applied to its respective input an amount determined by a signal applied to its respective control input, the signal generator further comprising a bias generator configured to receive the control signal and provide a bias signal to the control inputs in accordance with the control signal.
 9. The signal generator of claim 8, wherein the plurality of delay elements include a plurality of groups of delay elements, wherein individual ones of the groups are configured to provide a different resolution of signal delay control, and wherein the bias generator is further configured to provide different bias signals to individual ones of the groups of delay elements.
 10. A device comprising: an input buffer configured to receive a periodic input signal; a multi-phase periodic signal generator coupled to the input buffer, the multi-phase periodic signal generator comprising: a delay line configured to delay a first signal applied to an input of the delay line, wherein the first signal is based at least in part on the periodic input signal, to generate a second signal, and configured to delay the second signal to generate a third signal, the delay line further configured to provide the first second and third signals to a respective plurality of taps; a duty cycle distortion tolerance delay element configured to receive the first signal and the third signal from the respective plurality of taps, the duty cycle distortion tolerance delay element configured to delay the first and third signals and output delayed first and third signals; and a phase detector configured to receive the delayed first signal, the second signal, and the delayed third signal, the phase detector configured to be enabled responsive to the second signal and when enabled to provide a signal indicative of a phase difference between the delayed first and delayed third signals; and a plurality of memory cells configured to receive the second and third signals.
 11. The device of claim 10, further comprising: a locking loop configured to receive the periodic input signal and output the first signal, wherein the locked loop is configured to generate the first signal by delaying the periodic input signal by a delay amount, wherein the locking loop is further configured to receive a feedback signal and adjust a delay of the delay line such that the first signal is in phase with a clock signal received by the plurality of memory cells.
 12. The device of claim 11, wherein the feedback signal comprises the first signal passed through an output model delay element having a delay amount substantially equal to an amount of delay between an input of the multi-phase periodic signal generator and the plurality of memory cells.
 13. The device of claim 11, wherein the multi-phase periodic signal generator further comprises a control signal generator configured to receive the signal indicative of the phase difference between the delayed first and delayed third signals and generate a control signal to adjust a delay of the delay line such that the first, second, and third signals have a predetermined phase relationship.
 14. The device of claim 13, wherein the predetermined phase relationship includes a 180 degree phase difference between the first and second signals and a 360 degree phase difference between the first and third signals.
 15. The device of claim 13, wherein the multi-phase periodic signal generator is configured to adjust the phase relationship between the first and third signals and the phase relationship between the first and second signals in a single adjustment.
 16. A method for generating a plurality of periodic signals having a predetermined phase relationship, including a first signal, a second signal, and a third signal, the method comprising: enabling a phase detector responsive to an edge of the second signal; delaying the first and second signals a compensation amount prior to measuring the phase difference between the first and third signals; measuring the phase difference between the first and third signals while the phase detector is enabled; adjusting a first delay used to generate the second signal from the first signal based on the phase difference; and adjusting a second delay used to generate the third signal from the second signal based on the phase difference.
 17. The method of claim 16, wherein said adjusting a first delay and said adjusting a second delay occur at least partially simultaneously.
 18. The method of claim 16, wherein said adjusting a first delay and adjusting a second delay comprises adjusting the first delay and adjusting the second delay such that the predetermined phase relationship is attained.
 19. The method of claim 16, wherein the compensation amount delays the first and third signals such that rising edges of the first and third signals occur while the phase detector is enabled.
 20. The method of claim 16, further comprising changing the compensation amount based, at least in part, on a frequency of the first signal.
 21. The method of claim 16, further comprising inverting the second signal prior to said enabling a phase detector, and wherein said measuring a phase difference comprises generating a first difference signal corresponding to a length of time the inverted second signal and the delayed first signal have a same logic level and generating a second difference signal corresponding to a length of time the inverted second signal and the delayed third signal have the same logic level.
 22. The method of claim 16, wherein said adjusting a first delay and said adjusting a second delay comprise generating a coarse control signal and a fine control signal.
 23. The method of claim 16, further comprising providing at least one of the first, second, or third signals to a memory cell.
 24. The method of claim 16, wherein said adjusting a first delay and said adjusting a second delay occur in a single adjustment. 